It is well known that if a load is placed on a mat or belt, the load can be moved across a flat surface by applying a horizontal force to the belt. A technique for moving light loads of up to 200 pounds using narrow belts having a width of up to 5 feet is commonly used and is often referred to as a slider bed. Typically, such belts are made of a reinforced polyester fabric.
A similar technique is also used in automatic truck loading and unloading systems to move heavy loads of about 15 to 30 tons. In order to move such heavy loads in an automatic truck loading and unloading system, belts having a width of about 7 feet to 8 and 1/2 feet are required. At this combination of belt width and load, the belt of an automatic truck loading and unloading system must exhibit certain key physical characteristics in order to move the load successfully. Prior to the present invention, the required physical characteristics have been achievable only in multi-ply belts, that is, belts having a number of layers of polyester textile fabric with a rubber or plastic compound between the layers, and in some cases, on the top or bottom of the belt. Such a multi-ply construction is complicated and expensive.
A load-moving system of the slider bed type may be used, for example on trucks or trailers for loading or unloading the vehicle through the back end thereof by means of a powered conveyor belt extending along the bed of the vehicle between front and rear wind-up rolls. Thus, the belt may be attached at its forward end to a front wind-up roll which is, for example, located internally of the truck or trailer adjacent its front wall. The rear end of the belt may be attached to a rear wind-up roll which may be, for example, located externally under the back end of the truck or trailer. When loading the vehicle, the belt is wound onto the front roll, by driving the front roll and, conversely, for unloading the truck or trailer, the belt is wound onto the back roll by driving this roll. An automatic truck or trailer load-moving system using a slider bed technique requires a belt between 7 feet and 8 and 1/2 feet wide in order to move a load of up to about 30 tons.
After testing of a full size trailer with various loads between 15 and 40 tons, it has been determined that the single most important key physical characteristic of the belt is low stretch. For example, for a belt having a width of 91 and 1/2 inches, bearing a load weight of 25 tons and having a belt stretch before moving the load of 8%, when a pull on the belt per inch of width in the amount of 210 pounds was exerted, the belt arched longitudinally and finally folded over down part of its length and tracked off to one side. Similarly, for a belt having a width of 82 inches, bearing a load weight of 24 tons and having a belt stretch of 5%, when a pull on the belt per inch of width of 240 pounds was exerted, the belt arched longitudinally and folded over down part of its length and tracked off to one side.
On the other hand, for belts having a belt stretch of 2% and 2 and 1/2%, respectively, under the same conditions, the belt stayed flat and tracked true during a complete unloading sequence. The belt used in evaluating the significance of belt stretch in the above examples was of the expensive construction type having plies of polyester textile fabric. During the above test, it was also observed that the belt having the greatest lateral stiffness took longer to fold than the belt with the least lateral stiffness. The lateral stiffness of the most successful belt was 101.7 g, as measured by the Gurley test method. The lateral stiffness of the less successful belts ranged from 35 g to 60 g.
It was further observed during testing that the lower the coefficient of friction between the sliding surface of the belt and the floor over which it was sliding, the flatter the belt sat and the better it tracked. A further desirable characteristic of the belt is that it have an abrasion resistant and cut resistant top.
Accordingly, it is an object of the present invention to provide a belt which does not suffer from the disadvantages of known belts.
It is a further object of the present invention to provide a novel polyester textile fabric of sufficient strength and low stretch such that a single layer or ply of the novel polyester fabric would provide a maximum of 3% stretch when subjected to a pull of 300 pounds per inch of belt width applied in a longitudinal direction.